linux-kernel - Re: [PATCH 02/31] Add debugger entry points for X86

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Date:	Thu, 28 Jan 2016 12:20:30 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeffrey Merkey <jeffmerkey@...il.com>, linux-kernel@...r.kernel.org
Cc:	tglx@...utronix.de, mingo@...hat.com, x86@...nel.org,
	akpm@...ux-foundation.org, linux.mdb@...il.com, dzickus@...hat.com,
	dyoung@...hat.com, mnfhuang@...il.com
Subject: Re: [PATCH 02/31] Add debugger entry points for X86

On 01/28/16 11:46, Jeffrey Merkey wrote:
> This patch series adds an export which can be set by system debuggers to
> direct the hard lockup and soft lockup detector to trigger a breakpoint
> exception and enter a debugger if one is active.  It is assumed that if
> someone sets this variable, then an breakpoint handler of some sort will
> be actively loaded or registered via the notify die handler chain.
> 
> This addition is extremely useful for debugging hard and soft lockups
> real time and quickly from a console debugger.
> 
> Signed-off-by: Jeffrey Merkey <jeffmerkey@...il.com>
> ---
>  arch/x86/include/asm/kdebug.h | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/arch/x86/include/asm/kdebug.h b/arch/x86/include/asm/kdebug.h
> index e5f5dc9..cbe9004 100644
> --- a/arch/x86/include/asm/kdebug.h
> +++ b/arch/x86/include/asm/kdebug.h
> @@ -3,6 +3,11 @@
>  
>  #include <linux/notifier.h>
>  
> +static inline void arch_breakpoint(void)
> +{
> +	asm("   int $3");
> +}
> +
>  struct pt_regs;
>  

Even though apparently gas changed "int $3" to "int3" the latter is
actually a different instruction (with subtly different behavior) and
this may very well be corrected in gas at a future date.

So please encode this as "int3".  It should also be "asm volatile" since
it has side effects unbeknownst to the compiler.

	-hpa